In recent years, there are problems caused by the heavy burden of plastic waste on the global environment, such as harmful effects on the ecosystem, emission generation of harmful gas during combustion, and global warming due to the large amount of heat generated by combustion. Biodegradable plastics have been actively developed as materials that can solve these problems.
Carbon dioxide generated by combustion of biodegradable plastics obtained from raw materials derived from plants is originally present in the air. Therefore, such combustion does not increase the amount of carbon dioxide in the atmosphere. This is referred to as “carbon neutral” and is regarded as important under The Kyoto Protocol that sets targets for reducing carbon dioxide emissions. Therefore, active use of such plant-derived biodegradable plastics is desired.
Recently, from the viewpoint of biodegradability and carbon neutral, aliphatic polyester which are produced by microorganism using plant-derived raw materials as carbon sources have received attention as biodegradable plastics. Particularly, polyhydroxyalkanoate (hereinafter, sometimes referred to as PHA) resins have received such attention. Among PHA resins, poly(3-hydroxybutyrate) homopolymer resins, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resins, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resins (hereinafter, sometimes referred to as P3HB3HH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) copolymer resins, polylactic acid, etc. have received such attention.
However, since PHA has a slow crystallization speed and has a glass transition temperature lower than room temperature (about 0 to 4° C.), it is necessary to lengthen the cooling time for solidification after heating and melting in the molding process, and therefore the productivity is low. Especially, when intending to produce fibers by melt-spinning using PHA, fibers sticking together or fiber adhesion to a roll occur because the resin solidifies slowly, and therefore, it is difficult to produce fibers stably and the resulting fibers are low in quality.
As a means for solving such problems caused by the slow crystallization speed, there has been disclosed a polyester fiber having a specific crystal structure obtained by melt-spinning of a polyester resin containing poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) at a specific take-up speed (see PTL 1). In this method, the increase in molecular orientation associated with the elongation deformation in the spinning line induces an increase in the crystallization speed, and therefore, solidification from crystallization is completed before the fiber reaches the take-up roll which results in the possible winding of the fiber at the take-up role and the improvement in the physical properties of resulting fibers. However, in mass production of fibers using a large winding machine, since the fibers cannot be winded at a high speed soon after starting, the fibers are usually threaded at a low speed of about 100 m/min to 2,000 m/min and the winding speed is increased gradually. When the speed reaches the production speed, winding of the product starts. Meanwhile, an instrument called “suction gun”, which sucks fibers pneumatically is used for threading fibers at the start of a production, but the running speed of fibers when sucking with a suction gun is usually as low as about 2,000 to about 4,000 m/min. Although the method disclosed in PTL 1 is excellent in productivity and physical properties of resulting fibers if the spinning at a take-up speed of 1,500 m/min to 7,000 m/min can be realized, this literature fails to disclose how to improve the workability at the start of the production, such as the stage of threading or the stage of increasing the winding speed before reaching at the above-mentioned production condition.
Disclosed as a prior example of a melt-spinning technology of poly(3-hydroxyalkanoate) is a cold stretching method: immediately after P3HB3HH is extruded from an extruder, P3HB3HH is cooled rapidly to its glass transition temperature Tg or less to prevent blocking of P3HB3HH filaments; and then, P3HB3HH is partially crystalized quickly at its glass transition temperature Tg or more (see PTL 2). According to this method, melt-spinning is carried out by subjecting a polymer that hardly crystallizes like P3HB3HH not to crystallization but to its glass state by cooling, and therefore sticking of fibers or adhesion of fibers to a roll does not occur regardless of the slow crystallization speed thereof and it is possible to produce fibers stably. However, for PHA, whose glass transition temperature is lower than room temperature, it is necessary to establish a low temperature environment using a freezer or something alike in order to rapidly cool PHA to its glass transition temperature or less. Therefore, energy consumption is large, and large-scale equipment is required. Thus, problems remain from the viewpoint of practical use.
Further, disclosed as another prior example is a production method of spinning hollow yarn or multilobal yarn of a biodegradable aliphatic polyester at high speed for a limited melt flow rate and spinning temperature (see PTL 3). This production method requires cooling before drawing, therefore it is constrained by facility. Moreover, factors of molecular structure, such as the copolymerization ratio of poly(3-hydroxyalkanoate) significantly affect the crystallizability and spinnability and the strength of the fibers obtained. However, this prior example does not disclose or suggest an appropriate copolymerization ratio.
Furthermore, disclosed as another prior example is a production method of melt-spinning of poly(lactic acid-co-ethylene glycol) at a speed of 4,000 m/min or more (see PTL 4). This method shows a better high-speed spinnability than that of melt-spinning of polylactic acid alone. However, since polylactic acid is easily hydrolyzed, copolymerization with a hydrophilic polyethylene glycol block, would make poly(lactic acid-co-ethylene glycol) more easily to be hydrolyzed. Therefore, it is difficult to control the amount of water absorption.
As another prior example, it is disclosed to blend pentaerythritol as a nucleating agent for the purpose of improving the crystallization of polyhydroxyalkanoate which crystallizes slowly (see PTL 5). This literature discloses processing methods such as injection molding, blow molding, and extrusion forming, but it fails to disclose or suggest about fibers.